I don't have access to the article, but I'd imagine the folks with the higher amounts of artificial sweetener consumption would correlate quite highly with folks who have obesity-related health issues associated with cognitive decline. The abstract says the extremes of caloric consumption were excluded, were these obesity-related health issues controlled for?

In adjusted analyses, people who consumed the highest total amounts of these sweeteners had a 62% faster decline in overall thinking and memory skills, equivalent to about 1.6 years of aging, than those consuming the lowest amount.